A fault tree analysis (FTA) is a technique for expanding a fault event into causative lower events by taking a logical sum (OR) or a logical product. (AND) thereof, so as to create a tree structure (hereinafter referred to as an “FT diagram”), and for extracting a major cause from the lower events to reexamine its design in order to prevent the occurrence of a fault. In creating the FT diagram, comprehensive knowledge and a high degree of specialization in its field are required, and therefore, technology for assisting the creation of the FT diagram is required (JP2009-289020A).
FIG. 15 is an example of the FT diagram. In this example, the fault event of “high power transmission loss by pulley” is analyzed. As this fault event is caused at the time of “large slip amount” or “large frictional force”, the “large slip amount” and the “large frictional force” are the lower events of the fault event, which have the logical sum relationship.
As the event of the “large frictional force” is caused at the time of “large reaction force” or “large coefficient of friction”, the “large reaction force” and the “Large coefficient of friction” are the lower events of the “large frictional force”, which have the logical sum relationship. As the event of the “large reaction force” is caused at the time of “large belt tension” the “large belt tension” is the lower event of the “large reaction force”.
The events of the “large slip amount”, the “large belt tension”, and the “large coefficient of friction” that cannot be expanded into further lower events, among the above-described events, are referred to as fundamental events, and it is necessary to examine measures against these fundamental events in order to prevent the occurrence of the fault event.
It should be noted that, in this example, higher events and the lower events are connected simply by lines, and no description is made whether the lower events on the same level have the logical sum relationship or the logical product relationship. The reason for this is that, as most of the lower events on the same level have the logical sum relationship, the higher events and the lower events, and the lower events on the same level are connected simply by the lines when the events have the logical sum relationship, and the description of “AND”, meaning the logical product, is made next to the line connecting the higher event and the lower event when the events have the logical product relationship.